Molecular cloning and heterologous expression of novel glucosyltransferases from tobacco cultured cells that have broad substrate specificity and are induced by salicylic acid and auxin

Identification of two 6'-deoxychalcone 4'-glucosyltransferase genes in dahlia (Dahlia variabilis)

MAIN CONCLUSION

Two glycosyltransferase genes belonging to UGT88 family were identified to have 6'-deoxychalcone 4'-glucosyltransferase activity in dahlia. 6'-Deoxychalcones (isoliquiritigenin and butein) are important pigments for yellow and orange to red flower color. 6'-Deoxychalcones are glucosylated at the 4'-position in vivo, but the genes encoding 6'-deoxychalcone 4'-glucosyltransferase have not yet been identified. In our previous study, it was indicated that snapdragon (Antirrhinum majus) chalcone 4'-O-glucosyltransferase (Am4'CGT) has isoliquiritigenin 4'-glucosylation activity. Therefore, to identify genes encoding 6'-deoxychalcone 4'-glucosyltransferase in dahlia (Dahlia variabilis), genes expressed in ray florets that shared high homology with Am4'CGT were explored. As a result, c34671_g1_i1 and c35662_g1_i1 were selected as candidate genes for 6'-deoxychalcone 4'-glucosyltransferases in dahlia. We conducted transient co-overexpression of three genes (c34671_g1_i1 or c35662_g1_i1, dahlia aldo-keto reductase1 (DvAKR1) or soybean (Glycine max) chalcone reductase5 (GmCHR5), and chili pepper (Capsicum annuum) MYB transcription factor (CaMYBA)) in Nicotiana benthamiana by agroinfiltration. Transient overexpression of c34671_g1_i1, DvAKR1, and CaMYBA resulted in increase in the accumulation of isoliquiritigenin 4'-glucosides, isoliquiritigenin 4'-O-glucoside, and isoliquiritigenin 4'-O-[6-O-(malonyl)-glucoside]. However, transient overexpression of c35662_g1_i1, DvAKR1, and CaMYBA did not increase accumulation of isoliquiritigenin 4'-glucosides. Using GmCHR5 instead of DvAKR1 showed similar results suggesting that c34671_g1_i1 has isoliquiritigenin 4'-glucosyltransferase activity. In addition, we conducted co-overexpression of four genes (c34671_g1_i1, c35662_g1_i1 or Am4'CGT, DvAKR1 or GmCHR5, CaMYBA, and chalcone 3-hydroxylase from dahlia). Accumulation of butein 4'-O-glucoside and butein 4'-O-[6-O-(malonyl)-glucoside] was detected for c35662_g1_i1, suggesting that c35662_g1_i1 has butein 4'-glucosyltransferase activity. Recombinant enzyme analysis also supported butein 4'-glucosyltransferases activity of c35662_g1_i1. Therefore, our results suggested that both c34671_g1_i1 and c35662_g1_i1 are 6'-deoxychalcone 4'-glucosyltransferases but with different substrate preference.

The role of signaling compounds in enhancing rice allelochemicals for sustainable agriculture: an overview

MAIN CONCLUSION

Plant phytotoxin synthesis is influenced by intricate signaling networks like jasmonic acid (JA) and salicylic acid (SA). These compounds not only induce allelochemical production but also aid weed suppression and plant immunity. (-)-Loliolide, JA, SA, and their derivatives trigger rice allelochemical synthesis and gene expression. Enhancing allelochemical synthesis in crops offers an alternative, reducing reliance on traditional herbicides for effective weed management. Rice (Oryza sativa L.) serves as a crucial staple food crop, nourishing over half of the global population, particularly in South Asia. Within rice plants, various secondary metabolites are produced, contributing to its nutritional value and providing energy to consumers. Over the last 5 decades, researchers have investigated 276 distinct types of secondary metabolites found in rice plants. These metabolites predominantly include phenolic acids, flavonoids, steroids, alkaloids, terpenoids, and their derivatives. The role of these secondary metabolites is to regulate the growth and development of the rice plant. In this research paper, we have focused on the allelopathic potential of rice, which involves its active defense strategy to suppress other species in its vicinity. This defense mechanism is regulated by plant signaling compounds. These signaling compounds enable rice plants to recognize and detect competitors, pathogens, and herbivores in their environment. As a response, the rice plants elevate the production of defensive secondary metabolites. One crucial aspect of rice allelopathy is the phenomenon of neighbor detection. Rice plants can sense the presence of neighboring plants and respond accordingly to establish their competitive advantage and ensure their survival. This paper specifically highlights the impact of exogenously applied signaling compounds, namely Methyl salicylate (MeSA) and Methyl Jasmonate (MeJA), on paddy rice. The aim is to provide deeper insights into the signaling mechanisms involved in rice allelopathy and how the exogenous application of signaling compounds influence the induction and regulation of defensive secondary metabolites in rice plants. Comprehensive analysis of various researchers' studies clearly reveals that the application of these elicitor compounds noticeably augments the allelopathic potential of rice, resulting in heightened accumulation of phenolic acid compounds. Expansion in more enlistment of phenolics may be because of expansion in the activities of enzymes, such as cinnamate 4-hydroxylase (C4H) and phenylalanine ammonia-lyase (PAL), the two main enzymes of the phenylpropanoid pathway, which are associated with allelopathic crop plants, and along this, they recognize the presence of weeds and react by expanding allelochemical focuses. Consequently, substantial endeavors have been dedicated in recent times to discover and characterize plant-derived signaling molecules. In bioassays conducted by Patni et al. in 2019, both competitive and non-competitive rice genotypes exhibited elevated phytotoxicity against Echino colona following treatment with MeSA. MeSA-treated rice plants displayed accelerated growth, increased yield, and concurrently demonstrated weed-suppressing properties. Published studies from 1976 to 2021 are reviewed in this paper. The study indicates that signaling compounds induce allelochemical concentrations, enhancing allelopathic activity. This insight may lead to development of novel herbicides for effective sustainable weed management.

Functional Characterization of UDP-Glycosyltransferases Involved in Anti-viral Lignan Glycosides Biosynthesis in Isatis indigotica

Isatis indigotica is a popular herbal medicine with its noticeable antiviral properties, which are primarily due to its lignan glycosides such as lariciresinol-4-O-β-D-glucoside and lariciresinol-4,4'-bis-O-β-D-glucosides (also called clemastanin B). UDP-glucose-dependent glycosyltransferases are the key enzymes involved in the biosynthesis of these antiviral metabolites. In this study, we systematically characterized the UGT72 family gene IiUGT1 and two UGT71B family genes, IiUGT4 and IiUGT71B5a, with similar enzymatic functions. Kinetic analysis showed that IiUGT4 was more efficient than IiUGT1 or IiUGT71B5a for the glycosylation of lariciresinol. Further knock-down and overexpression of these IiUGTs in I. indigotica's hairy roots indicates that they play different roles in planta: IiUGT71B5a primarily participates in the biosynthesis of coniferin not pinoresinol diglucoside, and IiUGT1 primarily participates in the biosynthesis of pinoresinol diglucoside, while IiUGT4 is responsible for the glycosylation of lariciresinol and plays a dominant role in the biosynthesis of lariciresinol glycosides in I. indigotica. Analysis of the molecular docking and site-mutagenesis of IiUGT4 have found that key residues for its catalytic activity are H373, W376, E397, and that F151 could be associated with substrate preference. This study elucidates the biosynthetic route of anti-viral lignan glycosides in I. indigotica, and provides the foundation for the production of anti-viral lignan glycosides via synthetic biology under the heterologous model.

Systematic analysis of MYB gene family in Acer rubrum and functional characterization of ArMYB89 in regulating anthocyanin biosynthesis

The v-myb avian myeloblastosis viral oncogene homolog (MYB) family of transcription factors is extensively distributed across the plant kingdom. However, the functional significance of red maple (Acer rubrum) MYB transcription factors remains unclear. Our research identified 393 MYB transcription factors in the Acer rubrum genome, and these ArMYB members were unevenly distributed across 34 chromosomes. Among them, R2R3 was the primary MYB sub-class, which was further divided into 21 sub-groups with their Arabidopsis homologs. The evolution of the ArMYB family was also investigated, with the results revealing several R2R3-MYB sub-groups with expanded membership in woody species. Here, we report on the isolation and characterization of ArMYB89 in red maple. Quantitative real-time PCR analysis revealed that ArMYB89 expression was significantly up-regulated in red leaves in contrast to green leaves. Sub-cellular localization experiments indicated that ArMYB89 was localized in the nucleus. Further experiments revealed that ArMYB89 could interact with ArSGT1 in vitro and in vivo. Overexpression of ArMYB89 in tobacco enhances the anthocyanin content of transgenic plants. In conclusion, our results contribute to the elucidation of a theoretical basis for the ArMYB gene family, and provide a foundation for further characterization of the biological roles of MYB genes in the regulation of Acer rubrum leaf color.

Improving Phenolic Total Content and Monoterpene in Mentha x piperita by Using Salicylic Acid or Methyl Jasmonate Combined with Rhizobacteria Inoculation

The effects of plant inoculation with plant growth-promoting rhizobacteria (PGPR) and those resulting from the exogenous application of salicylic acid (SA) or methyl jasmonte (MeJA) on total phenolic content (TPC) and monoterpenes in Mentha x piperita plants were investigated. Although the PGPR inoculation response has been studied for many plant species, the combination of PGPR and exogenous phytohormones has not been investigated in aromatic plant species. The exogenous application of SA produced an increase in TPC that, in general, was of a similar level when applied alone as when combined with PGPR. This increase in TPC was correlated with an increase in the activity of the enzyme phenylalanine ammonia lyase (PAL). Also, the application of MeJA at different concentrations in combination with inoculation with PGPR produced an increase in TPC, which was more relevant at 4 mM, with a synergism effect being observed. With respect to the main monoterpene concentrations present in peppermint essential oil (EO), it was observed that SA or MeJA application produced a significant increase similar to that of the combination with rhizobacteria. However, when plants were exposed to 2 mM MeJA and inoculated, an important increase was produced in the concentration on menthol, pulegone, linalool, limonene, and menthone concentrations. Rhizobacteria inoculation, the treatment with SA and MeJA, and the combination of both were found to affect the amount of the main monoterpenes present in the EO of M. piperita. For this reason, the expressions of genes related to the biosynthesis of monoterpene were evaluated, with this expression being positively affected by MeJA application and PGPR inoculation, but was not modified by SA application. Our results demonstrate that MeJA or SA application combined with inoculation with PGPR constitutes an advantageous management practice for improving the production of secondary metabolites from M. piperita.

Glucosylation of the phytoalexin N-feruloyl tyramine modulates the levels of pathogen-responsive metabolites in Nicotiana benthamiana

Enzyme promiscuity, a common property of many uridine diphosphate sugar-dependent glycosyltransferases (UGTs) that convert small molecules, significantly hinders the identification of natural substrates and therefore the characterization of the physiological role of enzymes. In this paper we present a simple but effective strategy to identify endogenous substrates of plant UGTs using LC-MS-guided targeted glycoside analysis of transgenic plants. We successfully identified natural substrates of two promiscuous Nicotiana benthamiana UGTs (NbUGT73A24 and NbUGT73A25), orthologues of pathogen-induced tobacco UGT (TOGT) from Nicotiana tabacum, which is involved in the hypersensitive reaction. While in N. tabacum, TOGT glucosylated scopoletin after treatment with salicylate, fungal elicitors and the tobacco mosaic virus, NbUGT73A24 and NbUGT73A25 produced glucosides of phytoalexin N-feruloyl tyramine, which may strengthen cell walls to prevent the intrusion of pathogens, and flavonols after agroinfiltration of the corresponding genes in N. benthamiana. Enzymatic glucosylation of fractions of a physiological aglycone library confirmed the biological substrates of UGTs. In addition, overexpression of both genes in N. benthamiana produced clear lesions on the leaves and led to a significantly reduced content of pathogen-induced plant metabolites such as phenylalanine and tryptophan. Our results revealed some additional biological functions of TOGT enzymes and indicated a multifunctional role of UGTs in plant resistance.

Piriformospora indica cell wall modulates gene expression and metabolite profile in Linum album hairy roots

Elicitation of Linum album hairy roots by Piriformospora indica cell wall induced the target genes and specific metabolites in phenylpropanoid pathway and shifted the amino acid metabolism toward the phenolic compound production. Plants have evolved complex mechanisms to defend themselves against various biotic stresses. One of these responses is the production of metabolites that act as defense compounds. Manipulation of plant cell cultures by biotic elicitors is a useful strategy for improving the production of valuable secondary metabolites. This study focused on hairy root culture of Linum album, an important source for lignans. The effects of cell wall elicitor extracted from Piriformospora indica on phenylpropanoid derivatives were evaluated to identify metabolic traits related to biotic stress tolerance. Significant increases in lignin, lignans; lariciresinol, podophyllotoxin, and 6-methoxy podophyllotoxin; phenolic acids: cinnamic acid, ferulic acid, and salicylic acid; flavonoids: myricetin, kaempferol, and diosmin were observed in response to the fungal elicitor. In addition, the gene expression levels of phenylalanine ammonia-lyase, cinnamyl alcohol dehydrogenase, cinnamoyl-CoA reductase, and pinoresinol-lariciresinol reductase significantly increased after elicitation. The composition of free amino acids was altered under the elicitation. Phenylalanine and tyrosine, as precursors of phenylpropanoid metabolites, were increased, but alanine, serine, and glutamic acid significantly decreased in response to the fungal elicitor, suggesting that the amino acid pathway may be shifted toward biosynthesis of aromatic amino acids and precursors of the phenylpropanoid pathway. These results provided evidence that up-regulation of genes involved in the phenylpropanoid pathway in response to the fungal elicitor resulted in enhanced metabolic responses associated with the protection in L. album. This approach can also be applied to improve lignan production.

Structured plant metabolomics for the simultaneous exploration of multiple factors

Multiple factors act simultaneously on plants to establish complex interaction networks involving nutrients, elicitors and metabolites. Metabolomics offers a better understanding of complex biological systems, but evaluating the simultaneous impact of different parameters on metabolic pathways that have many components is a challenging task. We therefore developed a novel approach that combines experimental design, untargeted metabolic profiling based on multiple chromatography systems and ionization modes, and multiblock data analysis, facilitating the systematic analysis of metabolic changes in plants caused by different factors acting at the same time. Using this method, target geraniol compounds produced in transgenic tobacco cell cultures were grouped into clusters based on their response to different factors. We hypothesized that our novel approach may provide more robust data for process optimization in plant cell cultures producing any target secondary metabolite, based on the simultaneous exploration of multiple factors rather than varying one factor each time. The suitability of our approach was verified by confirming several previously reported examples of elicitor-metabolite crosstalk. However, unravelling all factor-metabolite networks remains challenging because it requires the identification of all biochemically significant metabolites in the metabolomics dataset.

Calcium mobilization in salicylic acid-induced Salvia miltiorrhiza cell cultures and its effect on the accumulation of rosmarinic acid

Ca(2+) serves as a second messenger in plant responses to different signals, and salicylic acid (SA) has been recognized as a signal mediating plant responses to many stresses. We recently found that SA treatment led to the cytoplasmic acidification of Salvia miltiorrhiza cells and alkalinization of extracellular medium. Here, we demonstrate that SA can rapidly induce Ca(2+) mobilization in protoplasts, but the induction can be blocked with a channel blocker of either plasma or organellar membranes. Following SA, A 23187, or 10 mmol/L Ca(2+) treatment, rosmarinic acid (RA) accumulation reached the highest level at 16 h, whereas the peak was found at 10 h if plasma membrane channel blockers were used. By contrast, the highest accumulation of RA occurred at 16 h when organellar channels were blocked, exhibiting the same tendency with SA-induced cells. In agreement with these observations, both phenylalanine ammonia-lyase (PAL) activity and its gene expression detected by real-time PCR also showed the same patterns. These results indicate that SA treatment firstly results in calcium release from internal stores, which in turn leads to PAL activity increase, RA accumulation, and a large amount of Ca(2+) influx from apoplast after 10 h of SA induction.

Production of salidroside in metabolically engineered Escherichia coli

Salidroside (1) is the most important bioactive component of Rhodiola (also called as “Tibetan Ginseng”), which is a valuable medicinal herb exhibiting several adaptogenic properties. Due to the inefficiency of plant extraction and chemical synthesis, the supply of salidroside (1) is currently limited. Herein, we achieved unprecedented biosynthesis of salidroside (1) from glucose in a microorganism. First, the pyruvate decarboxylase ARO10 and endogenous alcohol dehydrogenases were recruited to convert 4-hydroxyphenylpyruvate (2), an intermediate of L-tyrosine pathway, to tyrosol (3) in Escherichia coli. Subsequently, tyrosol production was improved by overexpressing the pathway genes, and by eliminating competing pathways and feedback inhibition. Finally, by introducing Rhodiola-derived glycosyltransferase UGT73B6 into the above-mentioned recombinant strain, salidroside (1) was produced with a titer of 56.9 mg/L. Interestingly, the Rhodiola-derived glycosyltransferase, UGT73B6, also catalyzed the attachment of glucose to the phenol position of tyrosol (3) to form icariside D2 (4), which was not reported in any previous literatures.

Genome-wide identification and tissue-specific expression analysis of UDP-glycosyltransferases genes confirm their abundance in Cicer arietinum (Chickpea) genome

UDP-glycosyltransferases (EC 2.4.1.x; UGTs) are enzymes coded by an important gene family of higher plants. They are involved in the modification of secondary metabolites, phytohormones, and xenobiotics by transfer of sugar moieties from an activated nucleotide molecule to a wide range of acceptors. This modification regulates various functions like detoxification of xenobiotics, hormone homeostasis, and biosynthesis of secondary metabolites. Here, we describe the identification of 96 UGT genes in Cicer arietinum (CaUGT) and report their tissue-specific differential expression based on publically available RNA-seq and expressed sequence tag data. This analysis has established medium to high expression of 84 CaUGTs and low expression of 12 CaUGTs. We identified several closely related orthologs of CaUGTs in other genomes and compared their exon-intron arrangement. An attempt was made to assign functional specificity to chickpea UGTs by comparing substrate binding sites with experimentally determined specificity. These findings will assist in precise selection of candidate genes for various applications and understanding functional genomics of chickpea.

Effects of methyl jasmonate and salicylic acid on tanshinone production and biosynthetic gene expression in transgenic Salvia miltiorrhiza hairy roots

Tanshinone is a group of active diterpenes, which are widely used in the treatment of cardiovascular disease. In this study, methyl jasmonate (MJ) and salicylic acid (SA) were used to investigate their effects on tanshinone accumulation and biosynthetic gene expression in the hairy roots of geranylgeranyl diphosphate synthase (SmGGPPS) overexpression line (G50) in Salvia miltiorrhiza. High-performance liquid chromatography analysis showed that total tanshinone content in G50 was obviously increased by 3.10-fold (11.33 mg/g) with MJ at 36 H and 1.63 times (5.95 mg/g) after SA treatment for 36 H in comparison with their mimic treatment control. Furthermore, quantitative reverse-transcription PCR analysis showed that the expression of isopentenyl-diphosphate delta-isomerase (SmIPPI), SmGGPPS, copalyl diphosphate synthase (SmCPS), and kaurene synthase-like (SmKSL) increased significantly with MJ treatment. However, the expression of SmIPPI reached the highest level at 144 H, whereas those of SmGGPPS, SmCPS, and SmKSL only increased slightly with SA treatment. The two elicitor treatments suggested that tanshinone accumulation positively correlated to the expression of key genes such as SmGGPPS, SmCPS, and SmKSL. Meanwhile, the study also indicated that it was a feasible strategy to combine elicitor treatment with transgenic technology for the enhancement of tanshinone, which paved the way for further metabolic engineering of tanshinone biosynthesis.

Hydrogen peroxide and nitric oxide are involved in salicylic acid-induced salvianolic acid B production in Salvia miltiorrhiza cell cultures

Hydrogen peroxide (H2O2) and nitric oxide (NO) are key signaling molecules in cells whose levels are increased in response to various stimuli and are involved in plant secondary metabolite synthesis. In this paper, the roles of H2O2 and NO on salvianolic acid B (Sal B) production in salicylic acid (SA)-induced Salvia miltiorrhiza cell cultures were investigated. The results showed that H2O2 could be significantly elicited by SA, even though IMD (an inhibitor of NADPH oxidase) or DMTU (a quencher of H2O2) were employed to inhibit or quench intracellular H2O2. These elicited H2O2 levels significantly increased NO production by 1.6- and 1.46 fold in IMD+SA and DMTU+SA treatments, respectively, and induced 4.58- and 4.85-fold Sal B accumulation, respectively. NO was also markedly elicited by SA, in which L-NNA (an inhibitor of NO synthase) and cPTIO (a quencher of NO) were used to inhibit or quench NO within cells, and the induced NO could significantly enhance H2O2 production by 1.92- and 1.37-fold in L-NNA+SA and cPTIO+SA treatments, respectively, and 3.27- and 1.50-fold for Sal B accumulation, respectively. These results indicate that elicitation of SA for either H2O2 or NO was independent, and the elicited H2O2 or NO could act independently or synergistically to induce Sal B accumulation in SA-elicited cells.

Molecular cloning and characterization of genistein 4'-O-glucoside specific glycosyltransferase from Bacopa monniera

Health related benefits of isoflavones such as genistein are well known. Glycosylation of genistein yields different glycosides like genistein 7-O-glycoside (genistin) and genistein 4'-O-glycoside (sophoricoside). This is the first report on isolation, cloning and functional characterization of a glycosyltransferase specific for genistein 4'-O-glucoside from Bacopa monniera, an important Indian medicinal herb. The glycosyltransferase from B. monniera (UGT74W1) showed 49% identity at amino acid level with the glycosyltransferases from Lycium barbarum. The UGT74W1 sequence contained all the conserved motifs present in plant glycosyltransferases. UGT74W1 was cloned in pET-30b (+) expression vector and transformed into E. coli. The molecular mass of over expressed protein was found to be around 52 kDa. Functional characterization of the enzyme was performed using different substrates. Product analysis was done using LC-MS and HPLC, which confirmed its specificity for genistein 4'-O-glucoside. Immuno-localization studies of the UGT74W1 showed its localization in the vascular bundle. Spatio-temporal expression studies under normal and stressed conditions were also performed. The control B. monniera plant showed maximum expression of UGT74W1 in leaves followed by roots and stem. Salicylic acid treatment causes almost tenfold increase in UGT74W1 expression in roots, while leaves and stem showed decrease in expression. Since salicylic acid is generated at the time of injury or wound caused by pathogens, this increase in UGT74W1 expression under salicylic acid stress might point towards its role in defense mechanism.

2-Oxoglutarate-dependent dioxygenases in the biosynthesis of simple coumarins

Coumarins are natural plant products that have been the subject of extensive phytochemical and pharmacological research studies in the past few decades. The core structure of coumarins is derived from the respective cinnamates via ortho-hydroxylation of the aromatic ring, trans/cis isomerization, and lactonization. Various substitution patterns of coumarins have been reported, whereas the biosynthesis of coumarins remains elusive. Ortho-hydroxylation is a key step in simple coumarin biosynthesis as a branch point from the lignin biosynthetic pathway. 2-Oxoglutarate-dependent dioxygenases (2OGDs) from plants convert cinnamate derivatives into simple coumarins through the process of ortho-hydroxylation. This review describes the 2OGDs involved in coumarin biosynthesis and their substrate specificities.

Molecular characterization of UGT94F2 and UGT86C4, two glycosyltransferases from Picrorhiza kurrooa: comparative structural insight and evaluation of substrate recognition

Uridine diphosphate glycosyltransferases (UGTs) are pivotal in the process of glycosylation for decorating natural products with sugars. It is one of the versatile mechanisms in determining chemical complexity and diversity for the production of suite of pharmacologically active plant natural products. Picrorhiza kurrooa is a highly reputed medicinal herb known for its hepato-protective properties which are attributed to a novel group of iridoid glycosides known as picrosides. Although the plant is well studied in terms of its pharmacological properties, very little is known about the biosynthesis of these important secondary metabolites. In this study, we identified two family-1 glucosyltransferases from P. kurrooa. The full length cDNAs of UGT94F4 and UGT86C4 contained open reading frames of 1455 and 1422 nucleotides, encoding polypeptides of 484 and 473 amino acids respectively. UGT94F2 and UGT86C4 showed differential expression pattern in leaves, rhizomes and inflorescence. To elucidate whether the differential expression pattern of the two Picrorhiza UGTs correlate with transcriptional regulation via their promoters and to identify elements that could be recognized by known iridoid-specific transcription factors, upstream regions of each gene were isolated and scanned for putative cis-regulatory elements. Interestingly, the presence of cis-regulatory elements within the promoter regions of each gene correlated positively with their expression profiles in response to different phytohormones. HPLC analysis of picrosides extracted from different tissues and elicitor-treated samples showed a significant increase in picroside levels, corroborating well with the expression profile of UGT94F2 possibly indicating its implication in picroside biosynthesis. Using homology modeling and molecular docking studies, we provide an insight into the donor and acceptor specificities of both UGTs identified in this study. UGT94F2 was predicted to be an iridoid-specific glucosyltransferase having maximum binding affinity towards 7-deoxyloganetin while as UGT86C4 was predicted to be a kaempferol-specific glucosyltransferase. These are the first UGTs being reported from P. kurrooa.

The molecular and enzymatic basis of bitter/non-bitter flavor of citrus fruit: evolution of branch-forming rhamnosyltransferases under domestication

Domestication and breeding of citrus species/varieties for flavor and other characteristics, based on the ancestral species pummelo, mandarin and citron, has been an ongoing process for thousands of years. Bitterness, a desirable flavor characteristic in the fruit of some citrus species (pummelo and grapefruit) and undesirable in others (oranges and mandarins), has been under positive or negative selection during the breeding process of new species/varieties. Bitterness in citrus fruit is determined by the composition of branched-chain flavanone glycosides, the predominant flavonoids in citrus. The flavor-determining biosynthetic step is catalyzed by two branch-forming rhamnosyltransferases that utilize flavanone-7-O-glucose as substrate. The 1,2-rhamnosytransferase (encoded by Cm1,2RhaT) leads to the bitter flavanone-7-O-neohesperidosides whereas the 1,6-rhamnosytransferase leads to the tastelessflavanone-7-O-rutinosides. Here, we describe the functional characterization of Cs1,6RhaT, a 1,6-rhamnosyltransferase-encoding gene directing biosynthesis of the tasteless flavanone rutinosides common to the non-bitter citrus species. Cs1,6RhaT was found to be a substrate-promiscuous enzyme catalyzing branched-chain rhamnosylation of flavonoids glucosylated at positions 3 or 7. In vivo substrates include flavanones, flavones, flavonols and anthocyanins. Cs1,6RhaT enzyme levels were shown to peak in young fruit and leaves, and gradually subside during development. Phylogenetic analysis of Cm1,2RhaT and Cs1,6RhaT demonstrated that they both belong to the branch-forming glycosyltransferase cluster, but are distantly related and probably originated separately before speciation of the citrus genome. Genomic data from citrus, supported by a study of Cs1,6RhaT protein levels in various citrus species, suggest that inheritance, expression levels and mutations of branch-forming rhamnosyltransferases underlie the development of bitter or non-bitter species/varieties under domestication.

Genes of phenylpropanoid pathway are activated in early response to Fusarium attack in flax plants

Fusarium is the most common flax pathogen causing serious plant diseases and in most cases leading to plant death. To protect itself, the plant activates a number of genes and metabolic pathways, both to counteract the effects of the pathogen, and to eliminate the threat. The identification of the plant genes which respond to infection is the approach, that has been used in this study. Forty-seven flax genes have been identified by means of cDNA subtraction method as those, which respond to pathogen infection. Subtracted genes were classified into several classes and the prevalence of the genes involved in the broad spectrum of antioxidants biosynthesis has been noticed. By means of semi-quantitative RT-PCR and metabolite profiling, the involvement of subtracted genes controlling phenylpropanoid pathway in flax upon infection was positively verified. We identified the key genes of the synthesis of these compounds. At the same time we determined the level of the metabolites produced in the phenylpropanoid pathway (flavonoids, phenolic acids) in early response to Fusarium attack by means of GC-MS technique. To the best of our knowledge this is the first report to describe genes and metabolites of early flax response to pathogens studied in a comprehensive way.

Comprehensive analysis of differential genes and miRNA profiles for discovery of topping-responsive genes in flue-cured tobacco roots

Decapitation/topping is an important cultivating measure for flue-cured tobacco, and diverse biology processes are changed to respond to the topping, such as hormonal balance, root development, source-sink relationship, ability of nicotine synthesis and stress tolerance. The purpose of this study was to clarify the molecular mechanism involved in the response of flue-cured tobacco to topping. The differentially expressed genes and micro RNAs (miRNAs) before and after topping were screened with a combination of suppression subtractive hybridization (SSH) and miRNA deep sequencing. In all, 560 differently expressed clones were sequenced by SSH, and then 129 high quality expressed sequence tags were acquired. These expressed sequence tags were mainly involved in secondary metabolism (13.5%), hormone metabolism (4%), signaling/transcription (17.5%), stress/defense (20%), protein metabolism (13%), carbon metabolism (7%), other metabolism (12%) and unknown function (13%). The results contribute new data to the list of possible candidate genes involved in the response of flue-cured tobacco to topping. NAC transcription factor, a differential gene identified by SSH, had been proved to have a role in the regulation of nicotine biosynthesis. High-throughput sequencing of two small RNA libraries in combination with SSH screening revealed 15 differential miRNAs whose target genes were identical to some differential genes identified in SSH, suggesting that miRNAs play a critical role in post-transcriptional gene regulation in the response of flue-cured tobacco to decapitation. Based on the role of these miRNAs and differential genes identified from SSH in response to topping, an miRNA mediated model for flue-cured tobacco in response to topping is proposed.

Elicitor-induced cellular and molecular events are responsible for productivity enhancement in hairy root cultures: an insight study

A wide range of external stress stimuli triggers a plant cell to undergo a complex network of reactions that ultimately lead to the synthesis and accumulation of secondary metabolites. These secondary metabolites help the plant to survive under stress challenge. The potential of biotic and abiotic elicitors for the induction and enhancement of secondary metabolite production in various culture systems including hairy root (HR) cultures is well-known. The elicitor-induced defense responses involves signal perception of elicitor by a cell surface receptor followed by its transduction involving some major cellular and molecular events including activation of major secondary message signaling pathways. This result in induction of gene expressions escorting to the synthesis of various proteins mainly associated with plant defense responses and secondary metabolite synthesis and accumulation. The review discusses the elicitor-induced various cellular and molecular events and correlates them with enhanced secondary metabolite synthesis in HR systems. Further, this review also concludes that combining elicitation with in-silico approaches enhances the usefulness of this practice in better understanding and identifying the rate-limiting steps of biosynthetic pathways existing in HRs which in turn can contribute towards better productivity by utilizing metabolic engineering aspects.

Characterization of glycosyltransferases responsible for salidroside biosynthesis in Rhodiola sachalinensis

Salidroside, the 8-O-β-D-glucoside of tyrosol, is a novel adaptogenic drug extracted from the medicinal plant Rhodiola sachalinensis A. Bor. Due to the scarcity of R. sachalinensis and its low yield of salidroside, there is great interest in enhancing production of salidroside by biotechnological manipulations. In this study, two putative UDP-glycosyltransferase (UGT) cDNAs, UGT72B14 and UGT74R1, were isolated from roots and cultured cells of methyl jasmonate (MeJA)-treated R. sachalinensis, respectively. The level of sequence identity between their deduced amino acid sequences was ca. 20%. RNA gel-blot analysis established that UGT72B14 transcripts were more abundant in roots, and UGT74R1 was highly expressed in the calli, but not in roots. Functional analysis indicated that recombinant UGT72B14 had the highest level of activity for salidroside production, and that the catalytic efficiency (Vmax/Km) of UGT72B14 was 620% higher than that of UGT74R1. The salidroside contents of the UGT72B14 and UGT74R1 transgenic hairy root lines of R. sachalinensis were also ∼420% and ∼50% higher than the controls, respectively. UGT72B14 transcripts were mainly detected in roots, and UGT72B14 had the highest level of activity for salidroside production in vitro and in vivo.

Quantitative influence of endogenous salicylic acid level on taxuyunnanine C biosynthesis in suspension cultures of Taxus chinensis

Intracellular signals are critical to secondary metabolite biosynthesis by plant cells, but their quantitative effects are not yet well understood in plant cell cultures. Using Taxus chinensis suspension culture, which has the potential to provide a sustainable supply of highly useful bioactive taxoids, this work investigated the impact of endogenous salicylic acid (SA) level on taxuyunnanine C (Tc) biosynthesis. It was observed that the Tc accumulation was strongly dependent on the endogenous SA level. Under elicitation with an endogenous SA of 97.1 ± 11.8 µg/g dry weight, a maximal Tc content of 10.3 ± 0.52 mg/g dry weight was obtained. The transcription levels of Tc biosynthetic genes of geranylgeranyl diphosphate synthase and taxadiene synthase were also analyzed, and they were increased with increase of internal SA level. The results demonstrated that manipulation of endogenous SA level could be an efficient strategy for improving secondary metabolite biosynthesis in plant cell culture.

UDP-glucose:3-deoxyanthocyanidin 5-O-glucosyltransferase from Sinningia cardinalis

3-Deoxyanthocyanins are rare anthocyanin pigments produced by some mosses, ferns, and higher plants. The enzymes and genes responsible for biosynthesis of 3-deoxyanthocyanins have not been well characterized. We identified a novel gene encoding UDP-glucose:3-deoxyanthocyanidin 5-O-glucosyltransferase (dA5GT) from Sinningia cardinalis, which accumulates abundant 3-deoxyanthocyanins in its petals. Five candidate genes (ScUGT1 to ScUGT5) were isolated from an S. cardinalis flower cDNA by degenerate PCR targeted for the UGT88 clade. ScUGT1, ScUGT3, and ScUGT5 exhibited 45-47% identity with rose anthocyanidin 5,3-O-glucosyltransferase, which catalyzes glucosylation at the 5- and 3-position of 3-hydroxyanthocyanidin. Based on its temporal and spatial gene expression patterns, and enzymatic activity assays of the recombinant protein, ScUGT5 was screened as a dA5GT candidate. Recombinant ScUGT5 protein expressed in Escherichia coli was used to analyze the detailed enzymatic properties. The results demonstrated that ScUGT5 specifically transferred a glucosyl moiety to 3-deoxyanthocyanidins in the presence of UDP-glucose, but not to other flavonoid compounds, such as 3-hydroxyanthocyanidins, flavones, flavonols, or flavanones.

Cloning and characterization of a glucosyltransferase from Crocus sativus stigmas involved in flavonoid glucosylation

BACKGROUND

Flavonol glucosides constitute the second group of secondary metabolites that accumulate in Crocus sativus stigmas. To date there are no reports of functionally characterized flavonoid glucosyltransferases in C. sativus, despite the importance of these compounds as antioxidant agents. Moreover, their bitter taste makes them excellent candidates for consideration as potential organoleptic agents of saffron spice, the dry stigmas of C. sativus.

RESULTS

Using degenerate primers designed to match the plant secondary product glucosyltransferase (PSPG) box we cloned a full length cDNA encoding CsGT45 from C. sativus stigmas. This protein showed homology with flavonoid glucosyltransferases. In vitro reactions showed that CsGT45 catalyses the transfer of glucose from UDP_glucose to kaempferol and quercetin. Kaempferol is the unique flavonol present in C. sativus stigmas and the levels of its glucosides changed during stigma development, and these changes, are correlated with the expression levels of CsGT45 during these developmental stages.

CONCLUSION

Findings presented here suggest that CsGT45 is an active enzyme that plays a role in the formation of flavonoid glucosides in C. sativus.

Transcriptome analysis of Arabidopsis roots treated with signaling compounds: a focus on signal transduction, metabolic regulation and secretion

Gene expression in response to signaling molecules has been well studied in the leaves of the model plant species Arabidopsis thaliana. However, knowledge of gene expression and metabolic regulation at the root level is limited. Here, the signaling compounds salicylic acid (SA), methyl jasmonate (MeJA) and nitric oxide (NO) were applied exogenously to induce various defense responses in roots, and their effect was studied using a combination of genomic, molecular and biochemical approaches. Genes involved in defense signaling/activation, cellular redox state, metabolism, transcription factors and membrane transport were altered in expression following treatment with SA, MeJA and NO. In addition, it was found that SA-, MeJA- and NO-elicited roots increased the root exudation of phytochemicals compared with the roots of nontreated control plants. Transport systems likely to be involved in the root exudation of phytochemicals, including the MATE, ABC, MFS, amino acid, sugar and inorganic solute transporters, showed altered expression profiles in response to treatments. Overall, significant differences were found in the signaling compound-elicited expression profiles of genes in roots vs those in leaves. These differences could be correlated to the underground nature of roots and their exposure to higher microbial inoculum rates under natural conditions.

Multi-substrate flavonol O-glucosyltransferases from strawberry (Fragaria x ananassa) achene and receptacle

In an effort to characterize fruit ripening-related genes functionally, two glucosyltransferases, FaGT6 and FaGT7, were cloned from a strawberry (Fragaria x ananassa) cDNA library and the full-length open reading frames were amplified by rapid amplification of cDNA ends. FaGT6 and FaGT7 were expressed heterologously as fusion proteins in Escherichia coli and target protein was purified using affinity chromatography. Both recombinant enzymes exhibited a broad substrate tolerance in vitro, accepting numerous flavonoids, hydroxycoumarins, and naphthols. FaGT6 formed 3-O-glucosides and minor amounts of 7-O-, 4'-O-, and 3'-O-monoglucosides and one diglucoside from flavonols such as quercetin. FaGT7 converted quercetin to the 3-O-glucoside and 4'-O-glucoside and minor levels of the 7- and 3'-isomers but formed no diglucoside. Gene expression studies showed that both genes are strongly expressed in achenes of small-sized green fruits, while the expression levels were generally lower in the receptacle. Significant levels of quercetin 3-O-, 7-O-, and 4'-O-glucosides, kaempferol 3-O- and 7-O-glucosides, as well as isorhamnetin 7-O-glucoside, were identified in achenes and the receptacle. In the receptacle, the expression of both genes is negatively controlled by auxin which correlates with the ripening-related gene expression in this tissue. Salicylic acid, a known signal molecule in plant defence, induces the expression of both genes. Thus, it appears that FaGT6 and FaGT7 are involved in the glucosylation of flavonols and may also participate in xenobiotic metabolism. The latter function is supported by the proven ability of strawberries to glucosylate selected unnatural substrates injected in ripe fruits. This report presents the first biochemical characterization of enzymes mainly expressed in strawberry achenes and provides the foundation of flavonoid metabolism in the seeds.

FaGT2: a multifunctional enzyme from strawberry (Fragaria x ananassa) fruits involved in the metabolism of natural and xenobiotic compounds

Fragaria x ananassa UDP-glucose:cinnamate glucosyltransferase (FaGT2) catalyzes the formation of cinnamic acid and p-coumaric acid glucose esters during strawberry fruit ripening. Here, the ripening and oxidative stress induced enzyme was further characterized by testing a range of structurally different substrates of natural and unnatural origin in vitro and comparing their kinetic parameters to elucidate its additional biological functions. The accepted substrates ranged from derivatives of cinnamic acid and benzoic acid to heterocyclic and aliphatic compounds resulting in the formation of O- and S-glucose esters, as well as O-glucosides. In planta assays confirmed the formation of glucose derivatives after injection of the substrates into strawberry fruits. Common chemical and structural features required for activity were the easy subtraction of a proton from the glucosylation site and the conjugation of the formed anion with pi-electrons as best realized in the simplest substrate sorbic acid. In addition to cinnamic acid, the natural compounds anthranilic acid, trans-2-hexenoic acid, nicotinic acid and 2,5-dimethyl-4-hydroxy-3[2H]-furanone were glucosylated in vitro. But FaGT2 was also capable of efficiently converting xenobiotic substances like the herbicide 2,4,5-trichlorophenol and the herbicide analogue 3,5-dichloro-4-hydroxybenzoic acid. The results suggest that FaGT2 is involved in the detoxification of xenobiotics in accordance to its induction by oxidative stress.

A single amino acid in the PSPG-box plays an important role in the catalytic function of CaUGT2 (Curcumin glucosyltransferase), a Group D Family 1 glucosyltransferase fromCatharanthus roseus

Curcumin glucosyltransferase (CaUGT2) isolated from cell cultures of Catharanthus roseus exhibits unique substrate specificity. To identify amino acids involved in substrate recognition and catalytic activity of CaUGT2, a combination of domain swapping and site-directed mutagenesis was carried out. Exchange of the PSPG-box of CaUGT2 with that of NtGT1b (a phenolic glucosyltransferase from tobacco) led to complete loss of enzyme activity in the resulting recombinant protein. However, replacement of Arg378 of the NtGT1b PSPG-box with cysteine, the corresponding amino acid in CaUGT2, restored the catalytic activity of the chimeric enzyme. Further site-directed mutagenesis revealed that the size of the amino acid side-chain in that particular site is critical to the catalytic activity of CaUGT2.

Rice allelopathy induced by methyl jasmonate and methyl salicylate

Methyl jasmonate (MeJA) and methyl salicylate (MeSA) are important signaling molecules that induce plant defense against insect herbivores and microbial pathogens. We tested the hypothesis that allelopathy is an inducible defense mechanism, and that the JA and SA signaling pathways may activate allelochemicals release. Exogenous application of MeJA and MeSA to rice (Oryza sativa L.) enhanced rice allelopathic potential and led to accumulation of phenolics, an increase in enzymatic activities, and gene transcription of phenylalanine ammonia-lyase (PAL) and cinnamate 4-hydroxylase (C4H), two key enzymes in the phenylpropanoid pathway. Aqueous extracts of the leaves of rice IAC165, a putative allelopathic variety, treated with MeSA (5 mM) or MeJA (0.05 mM), showed increased inhibitory effects (25 and 21%, respectively) on root growth of barnyardgrass (Echinochloa crus-galli L.), and increased inhibitory effects (18 and 23%, respectively) on shoot growth. Aqueous extracts from leaves of Huajingxian 1 rice, a putative nonallelopathic variety treated with MeJA and MeSA, caused 63 and 24% inhibition of root growth in barnyardgrass seedlings. The root exudates of both IAC165 and Huajingxian 1 plants treated with MeJA and MeSA for 48 hr also showed significant increases in their inhibitory effects on root growth of barnyardgrass seedlings. At the four-leaf stage, levels of 3,4-hydroxybenzoic acid, vanillic acid, coumaric acid, and ferulic acid that accumulated in the leaves were 5.3-, 31.3-, 2.2-, and 1.7-fold higher in response to MeJA exposure, and 3.3-, 13.1-, 2.0-, and 2.2-fold higher in response to MeSA. Treatments of MeSA and MeJA enhanced the PAL activity in the rice leaves up to 52.3 and 80.1%, respectively, whereas C4H activity was increased by 40.2 and 67%. Gene transcription of PAL and C4H in rice leaves significantly increased after the plants were subjected to treatment with MeJA and MeSA. These results suggest that allelopathy may be an active defense mechanism, and that plant signaling compounds are potentially valuable in its regulation.

Trifluralin herbicide-induced resistance of melon to fusarium wilt involves expression of stress- and defence-related genes

SUMMARY To identify genes involved in trifluralin herbicide-induced resistance of melon to Fusarium oxysporum f. sp. melonis, suppression subtractive hybridization (SSH) and cDNA-amplified fragment-length polymorphism (cDNA-AFLP) were used. A total of 123 clones-60 of which have never been isolated from melon-were isolated, sequenced and annotated. A significant proportion (35%) of the total 123 clones exhibited similarity to genes that have been formerly described as stress- or defence-related. Thirty-two selected clones were subjected to a detailed expression analysis, one-third of which were found to be up-regulated in response to trifluralin treatment and/or fusarium inoculation. The putative roles of seven of these clones in stress are discussed. Furthermore, the expression of four stress-related and up-regulated genes was enhanced when the plants were subjected to salinity stress, suggesting that trifluralin induces a general stress response which protects the plant against fusarium wilt.

Phenylpropanoid glycosyltransferases from osage orange (Maclura pomifera) fruit

Flavonoids and isoflavonoids are well known for their beneficial effects on human health and their anti-insect and anti-microbial activities in plants. Osage orange fruit is rich in prenylated isoflavones and dihydrokaempferol and its glucoside. Four glycosyltransferases were identified from a collection of osage orange fruit expressed sequence tags. Biochemical characterization suggested that the glycosyltransferase UGT75L4 might be responsible for glucosylation of dihydrokaempferol in vivo, although this enzyme exhibited broad substrate recognition toward isoflavonoids and flavonoids in vitro. UGT88A4 was active on coumarin substrates. Identification of highly active phenylpropanoid glycosyltransferases will facilitate the metabolic engineering of glycosylated natural products in plants.

Salicylic Acid Enhances Jaceosidin and Syringin Production in Cell Cultures of Saussurea medusa

Addition of 20 muM salicylic acid to Saussurea medusa cell cultures at day 6 resulted in jaceosidin and syringin productions up to 95 mg l(-1 )and 631 mg l(-1) which were, respectively, about 2.5- and 2.7-fold higher than in the control. The biomass was increased from 8 to 12 g l(-1). Expression of chalcone synthase gene (chs) increased sharply after 12 h treatment and was sustained up to 48 h; chalcone isomerase gene (chi) expression reached a peak at 24 h and decreased after 48 h; and phenylalanine ammonia-lyase activity increased by 7.5-fold (96 U mg(-1) protein) higher than in the control after 24 h. These results indicate that salicylic acid enhances the production of jaceosidin and syringin which is accompanied by induction of the related phenylpropanoid biosynthetic enzymes.

Cinnamate metabolism in ripening fruit. Characterization of a UDP-glucose:cinnamate glucosyltransferase from strawberry

Strawberry (Fragaria x ananassa) fruit accumulate (hydroxy)cinnamoyl glucose (Glc) esters, which may serve as the biogenetic precursors of diverse secondary metabolites, such as the flavor constituents methyl cinnamate and ethyl cinnamate. Here, we report on the isolation of a cDNA encoding a UDP-Glc:cinnamate glucosyltransferase (Fragaria x ananassa glucosyltransferase 2 [FaGT2]) from ripe strawberry cv Elsanta that catalyzes the formation of 1-O-acyl-Glc esters of cinnamic acid, benzoic acid, and their derivatives in vitro. Quantitative real-time PCR analysis indicated that FaGT2 transcripts accumulate to high levels during strawberry fruit ripening and to lower levels in flowers. The levels in fruits positively correlated with the in planta concentration of cinnamoyl, p-coumaroyl, and caffeoyl Glc. In the leaf, high amounts of Glc esters were detected, but FaGT2 mRNA was not observed. The expression of FaGT2 is negatively regulated by auxin, induced by oxidative stress, and by hydroxycinnamic acids. Although FaGT2 glucosylates a number of aromatic acids in vitro, quantitative analysis in transgenic lines containing an antisense construct of FaGT2 under the control of the constitutive 35S cauliflower mosaic virus promoter demonstrated that the enzyme is only involved in the formation of cinnamoyl Glc and p-coumaroyl Glc during ripening.

Glycosyltransferases of lipophilic small molecules

Glycosyltransferases of small molecules transfer sugars to a wide range of acceptors, from hormones and secondary metabolites to biotic and abiotic chemicals and toxins in the environment. The enzymes are encoded by large multigene families and can be identified by a signature motif in their primary sequence, which classifies them as a subset of Family 1 glycosyltransferases. The transfer of a sugar onto a lipophilic acceptor changes its chemical properties, alters its bioactivity, and enables access to membrane transporter systems. In vitro studies have shown that a single gene product can glycosylate multiple substrates of diverse origins; multiple enzymes can also glycosylate the same substrate. These features suggest that in a cellular context, substrate availability is a determining factor in enzyme function, and redundancy depends on the extent of coordinate gene regulation. This review discusses the role of these glycosyltransferases in underpinning developmental and metabolic plasticity during adaptive responses.

Metabolic engineering of plant cells for biotransformation of hesperedin into neohesperidin, a substrate for production of the low-calorie sweetener and flavor enhancer NHDC

Neohesperidin dihydrochalcone (NHDC) is a seminatural, safe, low-calorie sweetener, bitterness blocker, and flavor enhancer with unique properties and applications for the food, beverage, pharmaceutical, and animal feed industries. Current production is limited by the availability of the substrate neohesperidin, a flavonoid that accumulates to significant levels only in the inedible bitter citrus species. We propose a process to convert hesperidin, a tasteless flavonoid extracted from orange peels that are abundant byproducts of the vast orange juice industry, into neohesperidin using metabolic engineering and biotransformation via three steps: (i) extraction of hesperidin from orange peels, (ii) hydrolysis of sugar moieties, and (iii) biotransformation of hesperidin hydrolysis products into neohesperidin. We overcame the current technological bottleneck in biotransformation of hesperidin hydrolysis products into neohesperidin using metabolically engineered plant cell cultures expressing a recombinant flavanone-7-O-glucoside-2-O-rhamnosyltransferase. A small-scale production experiment established the feasibility of the proposed process.

Elicitor signal transduction leading to production of plant secondary metabolites

Plant secondary metabolites are unique sources for pharmaceuticals, food additives, flavors, and other industrial materials. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Understanding signal transduction paths underlying elicitor-induced production of secondary metabolites is important for optimizing their commercial production. This paper summarizes progress made on several aspects of elicitor signal transduction leading to production of plant secondary metabolites, including: elicitor signal perception by various receptors of plants; avirulence determinants and corresponding plant R proteins; heterotrimeric and small GTP binding proteins; ion fluxes, especially Ca2+ influx, and Ca2+ signaling; medium alkalinization and cytoplasmic acidification; oxidative burst and reactive oxygen species; inositol trisphosphates and cyclic nucleotides (cAMP and cGMP); salicylic acid and nitric oxide; jasmonate, ethylene, and abscisic acid signaling; oxylipin signals such as allene oxide synthase-dependent jasmonate and hydroperoxide lyase-dependent C12 and C6 volatiles; as well as other lipid messengers such as lysophosphatidylcholine, phosphatidic acid, and diacylglycerol. All these signal components are employed directly or indirectly by elicitors for induction of plant secondary metabolite accumulation. Cross-talk between different signaling pathways is very common in plant defense response, thus the cross-talk amongst these signaling pathways, such as elicitor and jasmonate, jasmonate and ethylene, and each of these with reactive oxygen species, is discussed separately. This review also highlights the integration of multiple signaling pathways into or by transcription factors, as well as the linkage of the above signal components in elicitor signaling network through protein phosphorylation and dephosphorylation. Some perspectives on elicitor signal transduction and plant secondary metabolism at the transcriptome and metabolome levels are also presented.

Microarray analysis of the gene expression profile induced by the endophytic plant growth-promoting rhizobacteria, Pseudomonas fluorescens FPT9601-T5 in Arabidopsis

Pseudomonas fluorescens FPT9601-T5 was originally identified as an endophytic plant growth-promoting rhizobacteria (PGPR) on tomato. To perform a molecular dissecttion of physiological and biochemical changes occurring in the host triggered by P. fluorescens FPT9601-T5 colonization, the model plant Arabidopsis was used in this study. Root colonization of Arabidopsis with P. fluorescens FPT9601-T5 promoted plant growth later than three weeks after inoculation and partially suppressed disease symptoms caused by Pseudomonas syringae pv. tomato DC3000, indicating that P. fluorescens FPT9601-T5 acted as a PGPR on Arabidopsis. To obtain a global view on transcript modification during the Arabidopsis-FPT9601-T5 interaction, we performed microarray analysis using Affymetrix Genechip probe arrays representing approximately 22,800 genes. The results showed that 95 and 105 genes were up- or down-regulated, respectively, more than twofold in FPT9601-T5-treated Arabidopsis plants as compared with control plants. Those up-regulated included genes involved in metabolism, signal transduction, and stress response. Noteworthy, upon FPT9601-T5 colonization, putative auxin-regulated genes and nodulin-like genes were up-regulated, and some ethylene-responsive genes were down-regulated. Our results suggest that P. fluorescens FPT9601-T5 triggered plant responses in a manner similar to known PGPR and, at least in some aspects, to rhizobia.

Molecular cloning, characterization, and downregulation of an acyltransferase that catalyzes the malonylation of flavonoid and naphthol glucosides in tobacco cells

Tobacco cells (Nicotiana tabacum L. Bright Yellow T-13) exposed to harmful naphthols accumulate them as glucosylated and further modified compounds [Taguchi et al. (2003a) Plant Sci. 164, 231-240]. In this study, we identified the accumulated compounds to be 6'-O-malonylated glucosides of naphthols. Cells treated with various phenolic compounds accumulated the flavonoids mainly as malonylglucosides. To clarify the function of this malonylation in tobacco, we isolated the cDNA encoding a malonyltransferase (NtMaT1) from a cDNA library derived from tobacco cells. The heterologous expression of the gene in Escherichia coli revealed that the recombinant enzyme had malonyltransferase activity against several phenolic glucosides such as flavonoid 7-O-glucosides, flavonoid 3-O-glucosides and naphthol glucosides. The substrate preference of the enzyme was similar to that of the tobacco cell extract. Malonylation activity in the transgenic cells markedly decreased with the suppression of the expression of NtMaT1 mRNA in tobacco BY-2 cells by RNA interference. The compounds administered to the transgenic cells were accumulated in the cells as glucosides or other modified compounds in place of malonylglucosides. These results show that NtMaT1 is the main catalyst of malonylation on glucosides of xenobiotic flavonoids and naphthols in tobacco plants.

Functional genomics uncovers three glucosyltransferases involved in the synthesis of the major sweet glucosides of Stevia rebaudiana

Stevia rebaudiana leaves accumulate a mixture of at least eight different steviol glycosides. The pattern of glycosylation heavily influences the taste perception of these intensely sweet compounds. The majority of the glycosides are formed by four glucosylation reactions that start with steviol and end with rebaudioside A. The steps involve the addition of glucose to the C-13 hydroxyl of steviol, the transfer of glucose to the C-2' and C-3' of the 13-O-glucose and the addition of glucose to the hydroxyl of the C-4 carboxyl group. We used our collection of ESTs, an UDP-glucosyltransferase (UGT)-specific electronic probe and key word searches to identify candidate genes resident in our collection. Fifty-four expressed sequence tags (ESTs) belonging to 17 clusters were found using this procedure. We isolated full length cDNAs for 12 of the UGTs, cloned them into an expression vector, and produced recombinant enzymes in Escherichia coli. An in vitro glucosyltransferase activity enzyme assay was conducted using quercetin, kaempferol, steviol, steviolmonoside, steviolbioside, and stevioside as sugar acceptors, and (14)C-UDP-glucose as the donor. Thin layer chromatography was used to separate the products and three of the recombinant enzymes produced labelled products that co-migrated with known standards. HPLC and LC-ES/MS were then used to further define those reaction products. We determined that steviol UGTs behave in a regioselective manner and propose a modified pathway for the synthesis of rebaudioside A from steviol.